High current laser diodes may be used for applications such as laser pumping and illumination. High current light emitting diodes (LEDs) may be used for applications such as general illumination, medical light sources, laboratory instruments.
Analog constant current sources or pulsed analog constant current sources using linear dissipative pass elements have been used as diode drivers to power light emitting diodes, often laser diodes. An array of LEDs (or diode array) are connected to a power source. A linear control (pass) element is disposed in the path between the LED array and the power source. Current flowing through the LEDs flows through a current sense resistor which supplies a voltage indicative of current to an input of an error amplifier, the other input of which receives a reference demand voltage indicative of the desired current. The output of the amplifier controls the linear control element to maintain a constant current through the LEDs. This is a simple, straightforward analog control loop. Such analog current sources are inefficient due to power (e.g., heat) dissipation in the linear pass element controlling the current. (See, e.g., FIG. 1 of U.S. Pat. No. 7,348,948, incorporated by reference herein.) An example of a linear pass element current source is the Model 778 Pulsed High Current Laser Diode Driver (Analog Modules, Inc., Longwood, Fla.). The 778 Series laser diode drivers are designed to power high current laser diodes, and may be used for pulsed or continuous wave (CW) LED or laser diode current source. Output currents of 1-100 A are available.
For pulsed laser or LED sources, the energy is typically stored in a capacitor to minimize a sudden lossy power demand from the prime power source. With a linear current regulator, the regulator pass element must remain in the linear region during the discharge of the energy storage capacitor to regulate the pulse of current. To minimize the voltage initially across this pass element, and hence dissipation, the capacitor must have a small value of voltage droop during this current draw, requiring a large amount of stored energy and a large capacitance value.
FIGS. 6A and 6B illustrate a buck converter of the prior art comprising a DC power supply “P”, a switch “S”, an inductor “L”, a load “R” and a diode “D”, connected as shown. The switch “S” may be a FET, and the load “R” may be a laser diode. The load “R” is grounded. The diode “D” is connected as a flyback, or freewheeling diode, via the load “R”, across the inductor “L”.
In FIG. 6A, the switch “S” is closed, and current flows through the inductor “L” and through the load “R”, but not through the diode “D”, as shown by the dashed-line arrow. In FIG. 6B, the switch “S” is opened, and current continues to flow through the inductor “L”, through the load “R”, and through the diode “D”, as shown by the dashed-line arrow.
FIGS. 7A and 7B a buck converter of the prior art comprising a DC power supply “P”, a switch “S”, an inductor “L”, a load “R” and a diode “D”, connected as shown. The switch “S” may be a FET, and the load “R” may be a laser diode. The load “R” is not grounded. The diode “D” is connected as a flyback, or freewheeling diode, via the load “R”, across the inductor “L”.
In FIG. 7A, the switch “S” is closed, and current flows through the load “R” and through the inductor “L”, but not through the diode “D”, as shown by the dashed-line arrow. In FIG. 7B, the switch “S” is opened, and current continues to flow through the load “R”, through the inductor “L”, and through the diode “D”, as shown by the dashed-line arrow.
In both of the examples given above, the buck converter operates asynchronously. By replacing the diode “D” with a (second) switch, operation may be made synchronous.
U.S. Pat. No. 5,287,372 (“Ortiz”) discloses a quasi-resonant diode drive current source that provides high power pulsed current that drives light emitting diodes, and the like. The pulsed output current of the quasi-resonant diode drive current source is sensed, and is regulated by a control loop to a level required by the light emitting diodes. In a specific embodiment of the invention, a zero-current-switched full wave quasi-resonant buck converter is described that provides a high amplitude pulsed output current required to drive light emitting pump diodes used in a solid state diode pumped laser. The use of a quasi-resonant converter as a pulsed current source provides a much higher conversion efficiency than conventional laser current sources. This higher efficiency results in less input power drawn from a power source and cooler operation, resulting in a higher reliability current source.
U.S. Pat. No. 5,736,881 (Ortiz), discloses a diode drive current source that uses a regulated constant current power source to supply current to drive a load, and the load current is controlled by shunt switches. If a plurality of loads utilize less than 50% duty factor, then one current source can drive N multiple dissimilar impedance loads, each at 100%/N duty factor. The current source includes a power converter coupled between the power source and the load(s) for providing pulsed current thereto. A current sensor is provided for sensing current flowing through the loads. A controller is coupled between the sensor and the power converter for regulating the amplitude of the output current supplied to the loads. A shunt switch is coupled across the loads, and a duty factor controller is coupled to the shunt switch for setting the duty factor of the shunt switch. A laser drive circuit, or driving light emitting diode arrays is also disclosed that include a plurality of the current sources. Alternatively, if the duty factor is sufficiently low, one current source may be used to drive a plurality of arrays.
U.S. Pat. No. 7,348,948 (“Crawford”), teaches a polyphase diode driver using multiple stages to generate a controlled current to the load. This approach may be efficient and have many advantages for military use, but may be somewhat complex for low cost commercial applications. More particularly,                A driver supplying a total current to a load has a plurality (n) of driver stages (ST1 . . . STn). One stage is a master stage. Each driver stage has a switching device (Q) and an inductor (L) connected in series between the switching device and the output of the driver stage. The switching devices are turned ON in sequence with one another, during a cycle time (Tc) which is determined by sensing current through the inductor (L1) in the master stage. When the switching device is turned ON current through the inductor rises, when the inductor current reaches the value of a demanded current the switch is turned OFF, and after the switch is turned OFF the inductor continues to supply (output) current to the load with a current which ramps down. A rectifying device (D) connected between the inductor and the supply line allows current to continue to flow in the inductor and be supplied to the load after the switch is turned OFF.        
U.S. Pat. No. 7,107,468, incorporated by reference herein, discloses a plurality of constant ON-time buck converters coupled to a common load. The output of each buck converter is coupled to a common load via a series sense resistor. The regulated output voltage across the common load is compared to a reference voltage to generate a start signal. The start signal is alternately coupled to the controller on each buck converter. The ON-time of a master buck converter is terminated when a ramp signal generated from the regulator input voltage exceeds the reference voltage. All other slave converters have an ON-time pulse started by the start signal and stopped by comparing a sense voltage corresponding to their output current during their ON-time pulse to the peak current in the master converter during its ON-time. A counting circuit with an output corresponding to each of the plurality of buck converters is used to select which buck converter receives the start signal. More particularly . . .                FIG. 1A is a simplified block diagram of a dual phase buck regulator with constant ON-time control and active current sharing. The output capacitor (C) 102 is usually a network of many capacitors in parallel. The equivalent series resistance (ESR) represented by resistor ESR 101 is the effective series resistance of this capacitor network. ESR 101 is the real part of the complex impedance of the network of parallel capacitors making up C 102. Two sense resistors, R 137 and R 103, provide voltages VR2 127 and VR1 122 that are proportional to the current in inductors 117 and 104 in each phase, respectively. VR1 122 is the difference in potential between node 124 and Vout 130 and VR2 127 is the difference between node 131 and Vout 130. The four field effect transistors (FETs), FET 106, FET 107, FET 116, and FET 118 control the duty cycle of each phase. Diodes 105 and 115 are flyback diodes that insure the currents in the inductors 104 and 117, respectively, are not interrupted. The gate drivers 119 and 120 in phase drive circuits 180 and 181 interface with the control circuit 121 and provide the voltages needed to drive FETs 106, 107, 116 and 118. The control circuit 121 determines which of the two phases, 180 or 181, to turn ON when the output voltage (Vout) 130 falls below the reference voltage (Vref) 123. The output currents IL1 141 and 1L2 142 combine to provide load current 1out 160 to load 140.        FIG. 1B illustrates the timing of two converter phases 180 and 181. The two graphs in FIG. 1B show that by complementary switching the two converter phases 180 and 181, both the amplitudes of the output current ripple (Iout 160) relative to output currents IL1 141 and IL2 142 and output voltage ripple (Vout 130) relative to sense voltages VR1 127 and VR2 122 are cut in half and the ripple frequency is doubled.        